The disclosure of Japanese Patent Application No. 2001-260024 filed on Aug. 29, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a non-isolated DCxe2x80x94DC converter for performing direct-current power conversion. In particular, the invention relates to a DCxe2x80x94DC converter suited to be connected on both input and output sides to charge/discharge units such as a battery and a capacitor.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2001-128369 discloses a DCxe2x80x94DC converter that is used in a state of being connected on both input and output sides to charge/discharge means. The DCxe2x80x94DC converter disclosed in this publication has two switching elements (MOS transistors having body diodes) connected to a reactor, and performs direct-current power conversion by holding one of the MOS transistors off and driving the other MOS transistor in an on/off manner. By interchanging the MOS transistor to be held off and the MOS transistor to be driven in an on/off manner, power can be charged in a bidirectional manner.
The DCxe2x80x94DC converter disclosed in this publication is advantageous in that power can be charged in a bidirectional manner. However, since the DCxe2x80x94DC converter is operated such that one of the MOS transistors is always held off, a current flows through a corresponding one of the body diodes in the normal direction. As a result, a problem is caused in respect of losses in the diode. For this reason, it is difficult to employ this DCxe2x80x94DC converter in a power circuit or the like in which losses in a diode raise a problem.
As a solution to such a problem, a DCxe2x80x94DC converter of the synchronous rectification control type in which two MOS transistors are operated in mutually inverted phases has been available as a non-insulated DCxe2x80x94DC converter designed to achieve the enhancement of efficiency by reducing losses in a diode.
On the other hand, such a non-insulated DCxe2x80x94DC converter is generally designed to perform soft-start control so as to prevent an overcurrent from flowing through an MOS transistor that is driven to be turned on at the beginning of an operation. According to soft-start control, the on-duty period is set short at first, is gradually extended afterwards, and reaches a desired period eventually.
However, if the DCxe2x80x94DC converter of synchronous rectification control type is connected on the output side to a power source, the on-duty period of the other MOS transistor is extended during soft-start control. As a result, an overcurrent flows through this MOS transistor. This is a contradiction to the original purpose of soft-start control. If an overcurrent flows through an MOS transistor, the MOS transistor may be destructed.
It is the object of the invention to improve a non-insolated DCxe2x80x94DC converter prevent in such a way that an overcurrent is prevented from flowing through a switching element during soft-start control.
The DCxe2x80x94DC converter according to the invention is a non-insulated DCxe2x80x94DC converter that comprises at least two switching elements connected to a coil and that is designed to perform direct-current power conversion by operating the at least two switching elements in mutually inverted phases by means of a control circuit. In order to solve the aforementioned problem, the control circuit performs soft-start control for gradually increasing an on-duty period of one of the switching elements at the beginning of an operation and holds the other switching element off during soft-start control.
The on-duty period of one of the switching elements is shorter during soft-start control than in a steady state. Besides, the other switching element is held off while soft-start control is performed. Thus, neither of the switching elements allows passage of an overcurrent.
Furtheron, the DCxe2x80x94DC converter may be so connected as to be located between a first charge/discharge unit and a second charge/discharge unit and may be designed to supply direct-current power from one of the charge/discharge units to the other. This DCxe2x80x94DC converter may be designed as follows. The switching elements include a first switching element and a second switching element. The first switching element is connected at one end to the first charge/discharge unit and at the other end to one end of the second switching element and one end of the coil. The coil is connected at the other end to the second charge/discharge unit.
If the DCxe2x80x94DC converter is thus configured, both step-up conversion and step-down conversion can be realized by suitably controlling the duty ratio at which a corresponding one of the switching elements is driven in an on/off manner.
Alternatively, the DCxe2x80x94DC converter may be so connected as to be located between a first charge/discharge unit and a second charge/discharge unit and may be designed to supply direct-current power from one of the charge/discharge units to the other. This DCxe2x80x94DC converter may be designed as follows. The first switching element is connected at one end to the first charge/discharge unit and at the other end to one end of the second switching element and one end of the coil. The coil is connected at the other end to one end of a third switching element and one end of a fourth switching element. The third switching element is connected at the other end to the second charge/discharge unit. The control circuit operates the fourth switching element in the same phase as the first switching element and operates the second and third switching elements in mutually inverted phases. At least one of the second and third switching elements is held off while soft-start control for gradually increasing an on-duty period of the first and fourth switching elements is performed at the beginning of an operation. At least one of the first and fourth switching elements is held off while soft-start control for gradually increasing an on-duty period of the second and third switching elements is performed at the beginning of an operation.
This DCxe2x80x94DC converter makes it possible to perform step-up conversion and step-down conversion in a bidirectional manner.